Crest voltmeter



p H. w. BOUSMAN' CREST VOLTMETER Filed Dec. -25, 1940 m mm a m moj 636m W A Im/ .s

' age system an Patented-sea. 9, 1941 UNITED STATES CREST VOLTMETER Henry W. Bousman, Scotia, N. Y., assignor to General Electric Company, a corporation oi New York I Application December 23, 1940, Serial No. 371,305

4 Claims. (cl. 121-95) My invention relates to apparatus formeasuring the crest voltage on high or medium voltage alternating current lines or apparatus. The in- V vention is particularly useful for measuring crest voltages usedin the high voltage testing of insulation because the stresses involved are proporticnal to the maximum rather than the root mean square value of the test voltage. An im-' portant object'of my invention is to provide relatively simple crest voltmeter equipment of high accuracy, and a second object of my invention is to provide a double range crest voltmeter of improved design.

The features of my invention which are believed to be novel and patentable will be point- ,ed out in the claims appended hereto.. For a better understanding of my invention reference .is made in the following description to the'accompanyi'ng drawing in which Fig. 1 represents vention; Fig. 2 is an equivalent circuit represen-% tation of the apparatus of Fig. 1; Fig. 3 represents a double range crest voltmeter embodying my invention; and Fig. 4 represents a vacuum tube voltmeter that maybe used with my invention.

In Fig.1, I represents a -conductor between which and ground a high alternating voltage exists. My crest voltmeter is shown connected to line l0 through the voltage reducing condenserconnection H and the insulation of a high voltage cable l2.

Included as a part of my crest voltmeterv at the point of "measurement is a condenser l3 connected between the crest voltmeter proper tion and its purpose is to materially reduce errors in measurement caused by any leakage resistance tha may exist between the high voltthe low voltage terminal which is generallygrounded as shown. The crest voltmeter propercomprises a condenser which serves both as a storage condenser and as' a part of a voltage divider, a one-way rectifier Ii for charging condenser l5 and a circuit containing resistance l1 and micro-ammeter l8 for measuring the condenser voltage. The leakage resistance referred to is that which is ordinarily due to leakage through the insulation from the high "voltage condenser ll; if used, to ground and through the insulation on the lead'or cable I! from condenser II to the place where the crest voltmeter is located. In Fig. 2 this leakage reii condenser I3 were omitted, the leakage resistance l9 would be connected directly in shunt to the condenser Iii/thereby materially increasing the error which is due to the generally un.-

avoidable presence of such leakage resistance.

The rectifier I6 is a half-wave rectifier of the vacuum tube heated cathode type. It by-passes the A, C. voltage wave of one polarity and the voltage wave of the opposite polarity flows into and charges the storage condenser I5 which thus tends to charge to a voltage proportional-to the crest voltage to be measured. This voltage is indicated by the instrument I8 connected across the condenser |5 through "a high resistance II. It will be noted that condenser 15 is here used both'as the storage condenser of the meter and as a part of the potential divider.

There are twdsources of error in the circuit I shown and the voltage developed across condenser I 5 willbe less than the crest voltage to be measured by the sum of such errors. One of these errors is caused by the discharge of con-- denser I5 through the instrument circuit which necessarily reduces the condenser voltage from the crest value which it would reach withou such discharge and is equal to 1 f gu is drawing and C15 is the capacitance of the storage condenser I5. Commercially available current measuring instruments of highsensitivity may 5 be used with commercially suitable capacitance divider capacitors without excessive error from this cause. The other'source of error is that whichis due to the presence of the leakage resistance [9 in shunt to a portion ofthe capacitance divider and is equal to Rl9 is the leakage resistance'and F g. 2. e

If in Fig. 2 the condenser l3 were omitted'so vthat theleakage resistance, I 9 *was directly in sistanhe is represented by l9. It is evident that shunt to the storage condenser I5, which has a 'not necessarily correspond.

Ordinarily this would increase the probable error 'materiallyand hence the presence'oI-condenser I3 to segregate the storage condenser I5 from any leakage resistance that may exist across sirable and it should be built in as a part of the crest voltmeter and be well insulated to prevent leakage. directly' across condenser I5. i It is. seen that -I have provided a crest voltmeter with inherent good accuracy and which' uses a relatively few standard elements, the

'condenser I5 being usedior the double purpose of -a-storage"condenser across which the crestvoltage is measuredand as'a part of the potential. divider employed.

Without confining my invention, as shown in Fig. 2 to any particular set-cf circuit constants,

(it may be helpful to setiorth one set of values for the different circuit elements, desirable when measuring peakvoltages on line I0 up to 100,000

volts at or above 60 cycles as follows: condenser II, .00025 microfarad; condenser I3, 1.0 microiaradyeondenser I5, 0.5 microiarad; resistance I'I, l'megohm with a micro-ammeter instrument at I8 requiring 50 micro-'amperes .for full scale deflection. The leakage resistance represented at I9 is'likely to have most any value greater than megohm. The error due to a megohm leakage resistance will be approximately 3%. If the same leakage resistance had been connected directlyacross condenser IS. the error would .be about %.v For measuring low voltages, say below 200 volts, the condenser II will be omitted.

' the terminals of the crest voltmeter circuit is der Condenser v 2,255,502 direct-current. voltage across it, the error due to its presence would be, J Y I a case. the connections would be as shown, using connection 22. The instrument l8 would read 25 and the reading multiplied by the factors 4 and 500, thus, 25X4X500=50,000. Fora 40,000 volt measurement we would use the external voltage divider but we would preferably use the terminal 23 of the crest voltmeter. In which case the reading of meter I8 would be 8 to be multiplied by N=500.

Now, in order to be able to use the double range crest voltmeter both with and without the ex- 'ternal voltage divider and retain the same ratio K- for either the high voltage and low voltage measurements such as has been given by way of example, it is necessary to-provide a certain relation between the various condensers included in the double ratio crest voltmeter arrangement, which includes the condensers 24 and 25, not heretofore mentioned. It may be stated,'however, that/if the external potential divider is never used, I could eliminate condenser 25 and still have-a double ratio crest voltmeter with the same ratio K. Condenser 25 is necessary only when used with an external potential divider and I the same ratioK both with and without the use of the external potential diit is desired to retain vider 26.

' 24 is chosen so that its capacitance bears such a relation to the capacitance of condensers I3 and I5 inseries thatth'e desired ratio K is obtained for the'tWO connections 22 and 23.

In Fig. 3 I have represented a double range crest voltmeter embodying my invention with a built-in potential divider. In Fig. 3 condensers 20 and 2I represent elements of an external potential divider designated 26 with my crest voltmeter connectedacross the condenser 2| thereof through the-insulated line I2. My double range crest voltmeter comprises the parts enclosed in the dotted line rectangle 21. Parts I3, I5, I0, II

in Fig. 2 although the circuit constant values do In Fig. 3 the incoming line may be connected to either terminal 22 or 23 to obtain two difierent measurement ratios.- For example, using the low-high voltage terminal 23 the instrument I8 may be calibrated directly in the peak voltage applied at 23. Then using the high-high voltage terminal 22, we'

- and the instrument I8 will be calibrated to read,

say 30. If the voltage to-be measured is 160 volts and the ratio K- is 4 the voltage will be applied to terminal 22, the instrument I8 will read 40 and is to be multiplied by 4. Ii. the voltage to be measured is 50,000 volts, for example, an exand I8 correspond to the parts of like number Condenser 25 is of such value that the capacitance, 0? the crest voltmeter 21.1Erom eitherterminals 22 or 23 to ground is the same, hence when using the potential divider 26, shifting the connection between 22 and 23 does not change the ratio with which the total voltage divides across the external potential divider and the crest voltmeter and hence neither K nor N changes.

The 'relation necessary for this result is obtained when 1 where C13, C24 and C25 are the values of the corresponding numbered condensers and K isthe ratio previously mentioned. An example of practicable values where K =4 is as follows:

Condenser 25=% microtarad Condenser =56 microfarad Condenser I3=1 microifarad Condenser, I5== microtarad' Resistance of the instrument circuit includingthat of instruments 18% megohm. The instrument at I8 may be a dArsonnal micro-ammeter requiring 50' mi'cro-amperes for full scaledeflection, which is obtained with 50 volts on ter- 'minal 23 and 200 volts -on terminal 22.

A satisfactory external voltage divider for use with such a crest voltmeter having a ratio N 01 500'and suitable for voltages up to 100,000 volts may comprise a condenser 20 of .001 microfarad and a condenser '2I of .082 microfarad.

In some cases where the amount of energy that'is available for measurement purposes is extremely small, connecting the crest voltmeter to the voltage source may result in lowering the voltage which it is desired to measure. This diiiiculty may be avoided if a voltmeter of very high resistance such as a. voltmeter oi the temal potential divided such as illustrated will a be used. It may have a ratio ll of 500. In such vacuum tubetype is used to measure the voltage across condenser II as represented at 30 in Flgr 4. Here the vacuum tube voltmeter is connected across condenser II through-a filter 282l.'

What I claim asnew and desire to secure by Letters Patent 0! the United States is:

l. A crest voltmeter circuit comprising high voltage and ground connecting terminals, a pair of condensers connected in series between said terminals, a rectifier valve and a high resistance measuring instrument circuit connected in parallel across the condenser which is connected to the ground terminal, said condenser serving as a storage condenser to supply the instrument circuit and as a part of a potential divided, the

condenser which is connected to the high voltage terminal serving as a part c! the same potential divider and for segregating the storage condenser from any leakage resistance that may exist be-. tween the high voltage and ground terminals.

, 2. A crest voltmeter circuit comprising high and low voltage terminals, a pair 0! condensers connected in series between said terminals, a rectifier and a high resistance instrument connected in parallel to the condenser which is connected to the low voltage terminal, said condenser serving as a storage condenser to-supply a direct current voltage to the instrument circuit and as a part of a potential divider and having a capacity 01 about half of that of the voltage terminal and the low voltage terminal,

a rectifier and a high resistance measuring instrument circuit connected across the condenser connected to the low voltage terminal, said last mentioned condenser serving as a storage condenser to supply a direct current voltage to the measuring instrument circuit and as apart of a potential divider and having a capacity greater condenser'connected to the high' voltage ter- Q minal, the last-mentioned condenser serving as a part or the same potential divider and for segregating the storage condenser from any leakage resistance that may exist across the ter-g minals 3. double range crest voltmeter'comprising low voltage terminal 'of such value that thecapacitance. of double range crest voltmeter high-high and lowshigh voltage terminals and a f low voltage terminal, the high voltage terminals being. separated by a condenser, a pair or coning circuit connected in parallel across the condenser which is connected to the low voltage terminal, said last mentioned condenser serving as a storage condenser to supply a direct current voltage to the instrument circuit and as a part of a potential divider and having -a capacitygreater than thefirst mentioned condenser but less than the other condenser 01 said pair; means for connecting measurement voltages between either ofv said high voltage terminals and said low voltage terminal and a condenser connected between the high-high voltage terminal and the between the voltage measurement connections is the same for. either of such connections. HENRY W. BOUSMAN.

densers connected in series between the low high 

